1. Field of the Invention
The present invention relates to a motor torque control device of an electric vehicle (including a hybrid vehicle), which causes a creeping phenomenon.
2. Description of Related Art
A creeping phenomenon usually occurs in a vehicle having an internal combustion engine with an automatic transmission, in which the vehicle attempts to move or creeps slightly forward due to the transmission of torque through a torque converter while the vehicle is stopped. In contrast, an electric vehicle is ordinarily capable of arbitrarily stopping without the occurrence of the creeping phenomenon.
This creeping phenomenon, however, has advantages of easily moving the vehicle forward at a creep speed, preventing the vehicle from moving backward even when a driver changes the position of the foot from a brake pedal to an accelerator pedal to start the vehicle on an uphill slope or in similar circumstances. For this reason, the creeping phenomenon is considered to be favorable in view of the driver""s driving operation. Since vehicles that are subject to the creeping phenomenon are widely used at present, vehicles that are not subject to the creeping phenomenon may give users a sense of incongruity.
Accordingly, an electric vehicle has been proposed that positively causes the creeping phenomenon. As is well known, an electric vehicle controls torque (driving torque) of a motor according to a torque command value set in accordance with an accelerator control input to thereby acquire a driving force in conformity with the operation of an accelerator. Therefore, when the accelerator control input is in the vicinity of zero, the torque command value is also zero. That is why a creep force usually cannot be acquired in an electric vehicle.
To address this problem, when the accelerator control input is in the vicinity of zero, a predetermined creep torque is set as the torque command value in order to acquire the creep force.
In this case, if the accelerator control input is increased from zero with the start of the vehicle, the torque command value is switched from the creep torque to a value (hereinafter referred to as acceleration torque) corresponding to the accelerator control input. A variety of ways have been proposed to control this switching operation. For example, the torque command value is switched from the creep torque to the acceleration torque when the accelerator control input is increased from zero. Alternatively, the torque command value is switched to the acceleration torque when the acceleration torque exceeds the creep torque.
In the former method, however, the acceleration torque is very close to zero at the moment the accelerator is pressed on. Thus, the torque command value is temporarily decreased from the creep torque to zero although the accelerator is operated. On a level road, a vehicle speed is lowered momentarily. On an uphill slope, the vehicle moves backward unexpectedly. It is therefore impossible to smoothly increase the vehicle speed. In the latter method, the predetermined creep torque is set as the torque command value until the acceleration torque exceeds the creep torque, and thus, the acceleration is delayed with respect to the operation of the accelerator, which gives an unnatural impression.
As a compromise between the above two methods, a value obtained by adding the creep torque to the acceleration torque is set as the power running torque command value until the acceleration torque exceeds the creep torque, and the torque command value is switched only to the acceleration torque when the acceleration torque exceeds the creep torque. More specifically, the power running torque command value reflects the acceleration torque from the start of the operation of the accelerator. This prevents the delay of the acceleration with respect to the operation of the accelerator. Since the torque command value is switched to the acceleration torque when the acceleration torque exceeds the creep torque, the torque command value can be prevented from being decreased to the vicinity of zero.
In the above compromise, however, the torque command value is decreased by the creep torque at the time of the switching. FIG. 2 shows the state wherein the vehicle is started on an uphill slope. In FIG. 2, the compromise is indicated by an alternate long and two short dashes line. As indicated by the alternate long and two short dashes line, the creep torque is decreased to zero at a point (a) when the acceleration torque exceeds the creep torque after the start of the vehicle, and therefore, the torque command value is sharply decreased. The decrease in the speed is less affected compared with the case where the creep torque is decreased to zero, because the torque command value, which is equal to the acceleration torque, still remains. However, it is impossible to smoothly increase the vehicle speed or to keep the vehicle from moving backward on a steep slope.
It is an object of the present invention to provide a motor torque control device of an electric vehicle, which enables smooth and natural acceleration in accordance with the operation of an accelerator at the start of the vehicle and the like.
The above object can be accomplished by providing an electric vehicle comprising: a shift position sensing device for sensing a shift position of a transmission mounted between a motor and driving wheels; a vehicle speed sensing device for sensing a vehicle speed; an accelerator sensing device for sensing an accelerator control input; a creep torque setting device for setting creep torque in accordance with a vehicle speed sensed by a vehicle speed sensing device when the shift position sensing device senses a running shift position an acceleration torque setting device for setting acceleration torque in accordance with the accelerator control input; a driving torque calculating device for calculating driving torque by adding the creep torque set by aid creep torque setting device to the acceleration torque set by the acceleration torque setting device; and a drive control device for running a motor in accordance with the driving torque calculated by the driving torque calculating device.